1. Field of the Invention
This invention generally relates to methods and systems for inspection of a wafer. Certain embodiments relate to a method for inspection of a wafer that includes illuminating the wafer with different wavelengths at least one of which penetrates into the wafer and at least one of which does not substantially penetrate into the wafer.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a specimen such as a semiconductor wafer using a number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that typically involves transferring a pattern to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and thus higher profits. Inspection has always been an important part of fabricating semiconductor devices such as integrated circuits. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the device to fail. For instance, as the dimensions of semiconductor devices decrease, detection of defects of decreasing size has become necessary since even relatively small defects may cause unwanted aberrations in the semiconductor devices.
In the past, it has been extremely difficult to discriminate between surface and subsurface defects on a wafer in a non-destructive manner (e.g., using an optical inspection system). Some approaches that have been used to determine if defects are surface or subsurface defects involve discriminating between crystal originated particles (COPs) and particle surface defects by comparing light scattered through different sets of solid angles and/or incidence angles.
One example of such a wafer inspection system for distinguishing between pits and particles is illustrated in U.S. Pat. No. 6,509,965 to Fossey et al., which is incorporated by reference as if fully set forth herein. The surface inspection system described by Fossey et al. includes a scanner configured to scan a P-polarized light beam across a surface of a workpiece. The system is configured to detect differences in the angular distribution of light scattered from the workpiece and to distinguish particle defects from pit defects based upon these differences. For example, if the defect is a pit, the amount of light scattered and detected by a center channel collector of the system is typically greater than that detected by a back channel collector of the system. Alternatively, if the defect is a particle, the amount of light detected by the center channel collector is typically less than that detected by the back channel collector and/or a forward channel collector of the system.
Another example of a system for discriminating between holes in and particles on a film covering a substrate is illustrated in U.S. Pat. No. 6,486,946 to Stover et al., which is incorporated by reference as if fully set forth herein. This system is configured to direct P-polarized light or light having a strong P-polarized component onto a filmed substrate at two (or more) different incidence angles, one of which is relatively large and the other of which is relatively small as measured from a surface normal. Light that is scattered into a back region of the hemispherical space above the substrate surface is collected and the intensity of the collected light is measured for each of the two incident angles. A defect can then be classified as either a hole in the film or a particle on the film based on the relatively intensities of the collected light. For instance, for holes formed in the film, the back-scattered light intensity at relatively large incidence (i.e., highly oblique incidence) is substantially smaller than the back-scattered light intensity at relatively small incidence (i.e., normal or near normal incidence). For particles, however, there is no such substantial decrease in intensity, and in many cases, the intensity actually increases slightly from small to large incidence. Accordingly, a defect can be classified as either a hole or a particle by measuring backscattered light intensity for both incidence angles and looking for a substantial decrease in intensity. If there is no such decrease, the defect is a particle. On the other hand, if there is a substantial decrease, the defect is a hole.
There are, however, a number of disadvantages to the currently available methods and systems for distinguishing between particle and pit defects. For example, two different types of COPs, one breaking the surface of the wafer and one totally submerged below the surface of the wafer, will both be classified as COPs by the systems and methods that are currently available. Of course, defect review tools and methods can be used to definitively determine if defects are surface or subsurface defects. For example, substantially slow methods such as focused ion beam (FIB) tools can be used to reveal the structure below the surface of the wafer. In addition, scanning electron microscopy (SEM) or atomic force microscopy (AFM) tools can be used to determine if a defect breaks the surface of the wafer, but these tools are also substantially slow. However, a definite need has been expressed for systems that are capable of discriminating between surface and subsurface defects as part of a standard wafer inspection with high accuracy and without using additional and/or substantially slow defect review tools.
Accordingly, it would be advantageous to develop methods and systems for inspection of a wafer that can be used to distinguish between surface and subsurface defects with relatively high accuracy and relatively high throughput.